Aluminum citrate is useful for crosslinking polymeric materials in subterranean formations to reduce the permeability of the formations to water. In oil producing formations, aluminum citrate has been used as a crosslinker for several years to improve oil recovery beyond what is recoverable by primary methods. Primary recovery involves production of the oil from the formation by natural driving forces such as gas expansion. Primary recovery normally only produces about one-third of the oil in the reservoir, leaving two-thirds of the oil still in place in the formation. Another one-third of the oil in the formation may be recovered by secondary or tertiary oil recovery techniques, referred to herein as improved oil recovery methods.
In an oil-bearing subterranean formation, oil, water, and sometimes gas, coexist in porous formation rock. Secondary recovery often involves injecting water into the formation via injection wells, in order to maintain pressure in the reservoir, and to drive the oil to producing wells. A disadvantage of using straight water to improve oil recovery is that water tends to move into the more highly permeable regions of rock, which are the easiest flow paths, and bypass the lower permeable rock. This results in uneven recovery of oil in the formation, where oil recovery is high in the highly permeable rock and the oil in the less permeable rock is left behind. Once the oil is recovered in the highly permeable zones, the zones become "watered-out" and increasing water is produced along with decreasing oil, making oil recovery uneconomic in a short time.
Secondary or tertiary oil recovery may involve adding a water-soluble polymer to the water injected into the formation in order to increase the viscosity of the water. This results in a more even flow of water into highly permeable and less permeable zones, and ultimately more oil recovery. The polymer performance can be improved substantially by crosslinking it after injection into the formation. Crosslinked polymer forms a gel in the formation rock in the highly permeable watered-out regions that selectively blocks these regions to additional flow of water. Because oil and water are immiscible, they occupy separate flow paths in the formation pores, so water flow in the rock is blocked to a much greater extent than oil flow with these water-soluble polymeric gels. The final result is higher oil recovery from the reservoir and less water production and recycling.
A material that is often used to crosslink polymeric materials in improved oil recovery operations is aluminum citrate. The trivalent aluminum metal acts as the active crosslinker for the polymer, and the citrate complexes the aluminum so that it is released slowly in the presence of the polymer after the solution is injected into the reservoir. A typical improved oil recovery operation may involve injection of a blended solution of dissolved water-soluble polymer and aluminum citrate into a reservoir over a period of several months.
A second area where gels are useful is in grouting operations. This would apply in certain construction projects where water encroachment is a problem. Grouts are injected into the subterranean formation to restrict the flow of water into the construction area. Grouts can also be used as a barrier to prohibit subterranean water movement in certain situations. For example, water encroachment on a foundation from a nearby pond might require use of a grout. Another use is in environmental remediation, where grouts can be used to temporarily block major water flow paths between a hazardous waste site and a potable water table.
When aluminum citrate was first used in the oilfield, according to U.S. Pat. No. 3,762,476 issued Oct. 2, 1973, aluminum sulfate hydrate and sodium citrate dihydrate were dry blended, then mixed with water at the desired concentration and pumped into the formation. There were several disadvantages of this method. First, the injected crosslinker solution had a very low pH, on the order of 2, which was corrosive to oilfield equipment. Second, the sulfate in the dry blend increased the tendency of certain oilfield waters to form sulfate scales. Third, the citrate and aluminum were not in contact for a sufficient time or in sufficient concentration for substantial chelation of the aluminum by the citrate to take place. As a result, aluminum release was more rapid, with premature and inconsistent gels formed too close to the wellbore, and aluminum lost to the formation via adsorption.
Manufacture of a liquid solution of aluminum citrate starting with aluminum chloride and aluminum sulfate is disclosed in British Patent No. 1,598,709. This liquid solution is unsuitable for improved oil recovery because the aluminum:citrate molar ratio is about 5.2:1, which results in unchelated aluminum in an acidic solution, with a pH of about 4 or less. The unchelated aluminum precipitates as the pH is increased to about 6.5, which is more suited for improved oil recovery. The maximum ratio of aluminum to citrate which allows for fully chelated aluminum is about 2.2:1.
An improvement over the original method of providing aluminum citrate to improved oil recovery projects was disclosed in U.S. Pat. No. 4,447,364. This method involves mixing a stable liquid aluminum citrate solution of pH 5.5 to 7.5 starting with aluminum chloride and citric acid. A 34 percent solution of aluminum chloride and a 50 percent solution of citric acid are blended such that the ratio of aluminum to citrate is from about 1.5:1 to 2:1. The pH of the acidic mixture is then adjusted upward with either ammonium hydroxide or sodium hydroxide. An intermediate aluminum citrate solution has an aluminum concentration of about 1 percent to 3 percent by weight. The final solution has an aluminum concentration of about 2.25 to about 3% by weight. The aluminum is fully chelated by the citrate. This is the current preferred solution for oilfield use. In certain situations, the practical use of this aluminum citrate solution is limited. Transportation of the liquid product to remote areas is expensive due to the relatively low active aluminum concentration. Ground transport is extremely expensive over long distances because the liquid product must be hauled in a tanker, which is usually full one way and empty on the return trip. Because the final product is a liquid, it must be either stored indoors or in a heated tank in extremely cold areas, where the temperature drops below about -20.degree. F. Therefore, desirable improvements over the current technology include increasing the active aluminum concentration in the solution and making a dry aluminum citrate product which can be more easily transported over long distances and stored under harsh environmental circumstances.
An alternative method of mixing liquid aluminum citrate, starting with sodium aluminate, is disclosed in U.S. Pat. No. 4,601,340. The citrate source is either sodium citrate or citric acid. The concentration of the final product may be from 3 to 3.5 percent by weight of aluminum, but the patent teaches that the aluminum concentration should be no greater than about 3 percent by weight.
Aluminum citrate compositions having a molar ratio of 1:1 aluminum to citrate have been disclosed, but this ratio is too low for practical use in gels because the aluminum is too tightly bound by the citrate, and therefore takes a long time to react with the polymer. U.S. Pat. Nos. 3,200,136 and 2,327,815 discuss the preparation of very dilute solutions with a 1:1 molar ratio of aluminum:citrate. The aluminum concentrations, which are much less than that provided by the current technology, are too low for practical use in subterranean formations. In U.S. Pat. No. 3,200,136, example 5 discloses a solid aluminum citrate material with an aluminum:citrate ratio of 2:1 which has a solution pH of about 3 at 10 percent solution, which would give a substantially lower pH in solutions having aluminum concentrations of 3 percent or more by weight. A pH of 3 is too low for practical use in gels. U.S. Pat. No. 2,327,815 discloses an aluminum citrate salt which is a 100 percent basic complex and does not have a chloride component. A solid aluminum citrate material made from aluminum nitrate and citric acid is discussed by Feng, et al., "Aluminum Citrate: Isolate and Structural Characterization of a Stable Trinuclear Complex," Inorg. Chem. (1990) 29:408-411, but again, the active aluminum concentration is too low for practical use.
Other liquid preparations comprising aluminum citrate in solution are known. See, e.g., Gallet, J.-P. and Paris, R. A., "Etude Thermometrique de la Formation des Complexes du Fer(III), de L'Aluminium et du Gallium," Anal. Chim. Acta (1967) 39:341-348; Weise, G. and Veith, J. A., "Komplexbidung Zwischen Zitronensaure und Aluminium," Z. Naturforsch. (1975) 30b:446-453; Karlik, S. J. et al., "Aluminum-27 Nuclear Magnetic Resonance Study of Aluminum(III) Interactions with Carboxylate Ligands," Inorg. Chem. (1983) 22:525-529; Ohman, L.-O. and Sjoberg, S., "Equilibrium and Structural Studies of Silicon(IV) and Aluminum(III) in Aqueous Solution. Part 9. A Potentiometric Study of Mono- and Poly-nuclear Aluminum(III) Citrates," J. Chem. Soc. Dalton Trans. (1983) pp. 2513-2517; Mak, M. K. S. and Langford, C. H., "Kinetic Analysis Applied to Aluminum Citrate Complexing," Inorg. Chim. Acta (1983) 70:237-246; Lopez-Quintela, M. A. et al., "Kinetics and Thermodynamics of Complex Formation Between Aluminum(III) and Citric Acid in Aqueous Solution," J. Chem. Soc. Faraday Trans. I (1984) 80:2313-2321; Gregor, J. E. and Powell, H. K. J., "Aluminum(III)-Citrate Complexes: a Potentiometric and .sup.13 C N.M.R. Study," Aust. J. Chem. (1986) 39:1851-1864; Ohman, L.-O., "Equilibrium and Structural Studies of Silicon(IV) and Aluminum(III) in Aqueous Solution. 17. Stable and Metastable Complexes in the System H.sup.+ --Al.sup.3+ -Citric Acid," Inorg. Chem. (1988) 27:2565-2570; Shioyama, T. K. and Little, R. A., U.S. Pat. Nos. 4,560,783; and 4,601,340. However, to Applicant's knowledge, no previous workers have been able to achieve stable aluminum citrate solutions having greater than 3.0 weight percent aluminum.
Aluminum citrate solutions have been used in oil recovery processes as described, e.g., in Mack, J., "Process Technology Improves Oil Recovery," Oil & Gas J. (1979) pp.67-71; Parmeswar, R. and Willhite, S., "A Study of the Reduction of Brine Permeability in Berea Sandstone With the Aluminum Citrate Process," SPE Reservoir Eng. (1988) pp. 959-966; and U.S. Pat. Nos. 3,762,476, 3,833,061, 3,952,806, 3,981,363, 4,018,286, 4,039,029, 4,120,361, 4,413,680, 4,488,601, 4,498,539, 4,526,231, 4,569,393, 4,579,176, 4,657,944, 4,644,193 and 5,151,615.
Aluminum citrate solutions have also been used in polymerization processes (as described in U.S. Pat. Nos. 3,533,973 and 3,839,255) and in pharmaceutical and related products (as described in U.S. Pat. Nos. 3,874,390, 3,924,642, 4,274,427, 4,645,662, 4,591,384, and 5,162,378). Miscellaneous industrial uses of aluminum citrate solutions include those described in U.S. Pat. Nos. 3,898,186, 3,910,805, 3,964,255, 4,116,931 and 4,612,175. Again, applicant is aware of no such solutions containing greater than 3.0 weight percent aluminum.
Recently a dry aluminum citrate material was developed by Haarman & Reimer, Elkhart, Indiana, with two samples privately submitted to the inventor for evaluation. The first sample contained 10.99 percent aluminum and 1 percent chloride, with about 8 percent of the material unaccounted for. The second sample contained 10.21 percent aluminum and 0.3 percent chloride, with about 14 percent of the material unaccounted for. The test results on these two dry aluminum citrate samples suggest that both materials were made with a source of aluminum which is relatively free of chloride. A source of aluminum which is in a chloride form is desirable for making an aluminum citrate product, because the chloride, which will be present in the final product, is generally a non-scaling ion when mixed with water prior to subterranean injection.
U.S. Pat. Nos. 3,294,860, 4,888,136, 4,898,842, 3,674,726 and 5,019,401, and Japanese Patent No. 92065802 mention the existence of an aluminum citrate product as a powder or salt; however, no methods for making such a product are taught. No commercial source of a dry aluminum citrate product has been found by applicant herein. References to commercial availability of such a product in the prior art appear to be erroneous.
All publications and patents referred to herein are incorporated by reference in their entirety.